Numerous behaviors, including alcoholism and depression are strongly sex- influenced in their prevalence and expression. In the rodent there is a critical sensitive period that extends into postnatal life during which steroids act on the brain to differentiate this target organ as "male" or "female". Exposure of the brain during this critical period will permanently determine the ability of the animal to ovulate or display sex- specific behaviors. The cellular mechanisms controlling the sexual differentiation of the brain in rodents are poorly understood. Other than the certainly that gonadal steroids binding to their intracellular receptors and activating them as specific transcription factors is importantly involved, little else is know. This project was designed to investigate what genes are being activated during the critical period of differentiation in response to steroid exposure and how that then leads to permanent structural alterations in the brain.